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GRE tunneling with the openvswitch plugin requires OVS kernel modules that are not part of the Linux kernel source tree. These modules are not available in certain Linux distributions, including Fedora and RHEL. Tunneling must not be configured on systems without the needed kernel modules. The Open vSwitch web site indicates the OVS GRE tunnel support is being moved into the kernel source tree, but patch ports are not. Once GRE support is available, it should be possible to support tunneling by using veth devices instead of patch ports.

Nova and Quantum currently assume all nodes where VMs are run will have access to all virtual networks. Cases where not all nodes have connections to all physical networks used with Quantum would be much better supported, if the Nova scheduler could query Quantum to determine whether a particular compute node supports the needed virtual networks.

Therefore, vlan is the preferred approach, if your configuration will support it. Otherwise, you might find yourself without all the troubleshooting tools available should something fail (and it always does).